Articles made from microcellular polymeric-foams use less material and have same or better mechanical properties than the unfoamed polymer. The combination of small cell size and high cell density is responsible for materials saving and enhanced properties. Microcellular foams usually have cell size of about 10 .mu.m and cell density of 10.sup.8 cells/cm.sup.3, and are stronger than conventional foams which have much bigger cells. It is thus desirable to discover ways to make foamed materials with even much smaller cells and much higher cell densities--materials with such characteristics are hereby termed as ultramicrocellular foams.